1. Field of the Invention
The present invention relates to a position detector which detects a coordinate position designated with a position pointer and which indicates the designated position on a coordinate display.
2. Related Art
A type of position detector is known which is designed to measure, with a processor, the period of time between a moment when a pulse is applied to one end of a magnetostriction transmitting medium or to a drive coil provided at the tip of a position pointer to generate a magnetostriction vibration wave in the magnetostriction transmitting medium and a moment when an induced voltage based on the magnetostriction vibration wave is detected across a detection coil provided at the tip of the position pointer or the end of the magnetostriction transmitting medium, thereby calculating the position designated with the position pointer. In another known position detector, a plurality of drive wires and a plurality of detection wires are arranged to be perpendicular to each other, and currents are successively supplied to the drive wires while the detection wires are successively selected to detect induced voltages, thereby enabling a position designated with a position pointer having a magnetic material such as ferrite to be detected from the position of detection wires from which higher induced voltage are detected.
The former detector is capable of detecting the designated position with comparatively high accuracy, but it requires a cable for exchange of a timing signal and so forth between the position pointer and the processor, and the handling of this detector is therefore considerably restricted. There is another problem in that the position pointer must be used by being maintained so as to be perpendicular to the magnetostriction transmitting medium while being close to the same. The latter device can be designed to make use of a cordless type of position pointer. In the latter device, however, the resolution with which the coordinate position is detected is determined depending upon the interval at which the wires are arranged. It is difficult to improve the resolution because if the interval is reduced in order to improve the resolution, the signal-to-noise ratio and the stability become deteriorated. It is also difficult to detect the position right above the point of intersection of each drive wire and each detection wire. Moreover, the position pointer must be positioned very close the wires.
Examples of position detectors having a display for indicating a designated position are also known: one in which a position detecting section (sensing section) having light transmission properties is superposed on a liquid crystal display; and one in which a liquid crystal display and a back light are superposed on a sensing section.
In the former example, however, it is necessary to space the position detecting section apart from the liquid display by at least 5 mm in order to avoid influence of noise generating from the liquid display. Correspondingly, the input surface of the tablet and the display surface of the liquid crystal display are spaced part from each other, and the operator perceives a difference between the movement of the tip of the position pointer and the locus of the same displayed on the liquid crystal and therefore becomes awkward in inputting operation.
In the latter example, there is a need for bringing the sensing section and the position pointer close to each other for desired coordinate inputting, and the back light and the liquid display are therefore designed to have very small thicknesses. Under this condition, a very thin light element such as an electroluminescent element (hereinafter referred to as "EL element") may be used as the back light but it is difficult to realize a display screen with a sufficient luminance by using the EL element.